The use of adsorptive processes in industry to separate various components from a gaseous mixture, e.g., oxygen from air is well known. Two major processes are currently in use. These are pressure swing adsorption (PSA) and vacuum swing adsorption (VSA). Pressure swing adsorption is carried out with the adsorption (feed) step at pressures much higher than ambient and adsorbent at pressures closer to an ambient. PSA processes are prone to high energy consumption when used to separate oxygen from air because the oxygen recovery is low and the entire feed train has to be compressed up to the adsorption pressure. The inefficiencies of the PSA process are somewhat minimized or circumvented by using vacuum swing adsorption. In the vacuum swing adsorption (VSA) processes, adsorption is carried at a pressure close to ambient and the adsorbent rate regeneration is carried out at sub-atmospheric pressure levels. The adsorbent beds go through several secondary steps with the primary aim of increasing oxygen recovery and reducing adsorbent inventory per unit of product gas.
A U.S. Pat. No. 4,561,865 illustrates a prior art pressure swing adsorption process.
Conventional PSA and VSA processes employ positive displacement compressors and blowers for either fluid compression or exhaustion in the adsorbent vessel. The conventional machines typically have lower efficiencies and higher maintenance costs than centrifugal compressors which are known in the art. In the past, attempts to utilize centrifugal compressors for PSA and VSA units have relied upon fixed speed centrifugal compressors with or without inlet guide vanes. However, centrifugal compressors operating at fixed speeds inherently have lower efficiencies at the lower pressure ratios when the pressure ratios change in a dramatic or dynamic fashion.
Positive displacement blowers currently used in, PSA and VSA systems exhibit three major drawbacks. First, positive displacement blowers show low efficiency in comparison to a centrifugal compressor at a single design point, with the difference in efficiency being as great as 15% in some cases. Positive displacement blowers tend to have problems including pressure pulsations, high sound level, and are prone to mechanical failures. Lastly, for large pressure ratios between the outlet and the inlet of the positive displacement blowers, two positive displacement blowers are needed in series with water injection into the gas in the positive displacement blowers for internal sealing and cooling of the blowers. The use of water injection adds to the overall cost of the system because of the cost of the water and the need to separate the water from the exhaust of the positive displacement blower.
The single largest problem with using a centrifugal compressor is the difficulty in reducing the pressure ratio in an efficient manner. The savings and efficiency in a VSA at full vacuum are lost during the times of low vacuum when a centrifugal compressor is employed. It has been proposed to increase the high efficiency range of a centrifugal compressor by installing guide vanes on the inlet compressor to act as a pre-whirl device to extend the range of vacuum level to where the centrifugal compressor is efficient. This device improves the efficiency to some degree, but not enough at the low pressure ratio part of the cycle. This device acts as an efficient device for flow decreasing, but not for lowering the pressure ratio. Such a technique is employed in the refrigeration industry as disclosed in U.S. Pat. Nos. 4,646,534 and 4,686,834, as well as in other process industries like air separation.
Variable speed driving (VSD) centrifugal compressors have been used in the past, however, usually in a slow changing environment such as temperature control. Variable speed driven compressors for refrigeration are shown in U.S. Pat. Nos. 4,417,452, 4,893,479, 5,123,080, and 5,214,367 which disclose various methods of variable frequency driving a compressor. Published International application PCT/US 94/01041 also discloses a variable speed motor for use with a compressor.